Exploring asymmetric substructures of the outer disk based on the conjugate angle of the radial action

TL;DR

This study uses the conjugate angle of radial action to analyze asymmetric structures in the outer disk, revealing ongoing phase mixing and linking features to past gravitational perturbations like the Sagittarius dwarf galaxy.

Contribution

It introduces the conjugate angle of radial action as a tool to identify and interpret asymmetric disk structures and their relation to phase mixing and external perturbations.

Findings

High aggregation of phase structures indicates ongoing phase mixing.

Absence of the Z-Vz phase spiral in Monoceros area.

Identification of a concentrated substructure linked to past gravitational events.

Abstract

We use the conjugate angle of radial action ($\theta_R$), the best representation of the orbital phase, to explore the "mid-plane, north branch, south branch" and "Monoceros area" disk structures that were previously revealed in the LAMOST K giants (Xu et al. 2020). The former three substructures, identified by their 3D kinematical distributions, have been shown to be projections of the phase space spiral (resulting from nonequilibrium phase mixing). In this work, we find that all of these substructures associated with the phase spiral show high aggregation in conjugate angle phase space, indicating that the clumping in conjugate angle space is a feature of ongoing, incomplete phase mixing. We do not find the $Z-V_Z$ phase spiral located in the "Monoceros area", but we do find a very highly concentrated substructure in the quadrant of conjugate angle space with the orbital phase from the apocenter to the guiding radius. The existence of the clump in conjugate angle space provides a complementary way to connect the "Monoceros area" with the direct response to a perturbation from a significant gravitationally interactive event. Using test particle simulations, we show that these features are analogous to disturbances caused by the impact of the last passage of the Sagittarius dwarf spheroidal galaxy.